Fluid-pressure operated punch presses are often used in metal forming operations. Such presses usually include a supporting frame and a fixedly mounted double-acting fluid actuator. A typical actuator used for operating a punch press includes a reciprocable piston movably mounted in an actuator cylinder, whereby a pair of fluid chambers are defined which can be selectively pressurized for "stroking" the piston in one direction or the other.
Fluid pressure, either hydraulic or pneumatic, is used to drive a punch or like tool carried by the piston rod of the actuator into the workpiece. To remove or strip the tool from the workpiece, it is necessary to clamp the workpiece temporarily. This is because the force required for removal of the tool is relatively high, typically about one-third of the force required by effecting punching. Known clamps have sometimes required additional power sources to activate them for gripping the workpiece. In addition, known clamp arrangements typically do not provide a large running clearance between the stripper and the workpiece. If the workpiece is bent or otherwise out of tolerance, it may not fit under the stripper clamp, thus requiring adjustment of the clamp, thereby detracting from the effeciency of the punching operation.
Accordingly, there continues to be a need for a stripper unit for a punch press that provides the greatest possible running clearance between the workpiece and the stripper, and that securely grips the workpiece during the return stroke of the press punch for stripping. Preferably, such an arrangement is configured such that it does not require additional actuation devices and can be operated attendant to fluid pressurization of the press double-acting actuator, and is configured for reliable and efficient operation.